The present invention relates to sensing body sounds, and more specifically, to acoustic-to-electrical transducers used for sensing body sounds, especially in stethoscopes.
Stethoscopes are widely used by health professionals to aid in the detection of body sounds. The procedures for listening to and analyzing body sounds, called auscultation, is often difficult to learn due to the typically low sound volume produced by an acoustic stethoscope. Electronic stethoscopes have been developed which amplify the faint sounds from the body. However, such devices suffer from distortion and ambient noise pickup. The distortion and noise are largely due to the performance of the acoustic-to-electrical transducers, which differ in operation from the mechanical diaphragms used in acoustic stethoscopes.
Acoustic stethoscopes have been the reference by which stethoscope sound quality has been measured. Acoustic stethoscopes convert the movement of the stethoscope diaphragm into air pressure, which is directly transferred via tubing to the listener""s ears. The listener therefore hears the direct vibration of the diaphragm via air tubes.
Existing electrical stethoscope transducers are typically one of three types: (1) microphones mounted behind the stethoscope diaphragm, or (2) piezoelectric sensors mounted on, or physically connected to, the diaphragm, or (3) other sensors which operate on the basis of electro-mechanical sensing of vibration via a sensing mechanism in mechanical contact with the diaphragm placed against the body
Microphones mounted behind the stethoscope diaphragm pick up the sound pressure created by the stethoscope diaphragm, and convert it to electrical signals. The microphone itself has a diaphragm, and thus the acoustic transmission path comprises stethoscope diaphragm, air inside the stethoscope housing, and finally microphone diaphragm. The existence of two diaphragms, and the intervening air path, result in excess ambient noise pickup by the microphone, as well as inefficient acoustic energy transfer. Various inventions have been disclosed to counteract this fundamentally inferior sensing technique, such as adaptive noise canceling, and various mechanical isolation mountings for the microphone. However, these methods are often just compensations for the fundamental inadequacies of the acoustic-to-electrical transducers.
The piezo-electric sensors operate on a somewhat different principle than merely sensing diaphragm sound pressure. Piezo-electric sensors produce electrical energy by deformation of a crystal substance. In one case, the diaphragm motion deforms a piezoelectric sensor crystal which is mechanically coupled to the stethoscope diaphragm, and an electrical signal results. The problem with this sensor is that the conversion mechanism produces signal distortion compared with sensing the pure motion of the diaphragm. The resulting sound is thus somewhat different in tone, and distorted compared with an acoustic stethoscope.
Other sensors are designed to transfer mechanical movement of the diaphragm, or other surface in contact with the body, via some fluid or physical coupling to an electromechanical sensing element. The problem with such sensors is that they restrict the mechanical movement of the diaphragm by imposing a mechanical load on the diaphragm. Acoustic stethoscopes have diaphragms that are constrained at the edges or circumference, but do not have any constraints within their surface area, other than the inherent elasticity imposed by the diaphragm material itself. Thus placing sensors in contact with the diaphragm restrict its movement and change its acoustic properties and hence the sounds quality capacitive acoustic sensors have been disclosed and are in common use in high performance microphones and hydrophones. A capacitive microphone utilizes the variable capacitance produced by a vibrating capacitive plate to perform acoustic-to-electrical conversion. Dynamic microphones that operate on the principle of a changing magnetic field are well-known. These devices typically operate by having a coil move through a static magnetic field, thereby inducing a current in the coil. Optical microphones have been disclosed, which operate on the principle that a reflected light beam is modified by the movement of a diaphragm.
A capacitive, magnetic or optical microphone placed behind a stethoscope diaphragm would suffer from the same ambient noise and energy transfer problems that occur with any other microphone mounted behind a stethoscope diaphragm. A unique aspect of the present invention is that the stethoscope diaphragm is the only diaphragm in the structure, whereas existing microphone-based solutions comprise a stethoscope diaphragm plus a microphone diaphragm, resulting in the inefficient noise-prone methods described previously.
The present invention provides both direct sensing of the diaphragm movement, with the diaphragm making direct contact with the body, while at the same time avoids any change in acoustic characteristics of the diaphragm compared with that of an acoustic stethoscope, since the sensing means does not mechanically load the diaphragm. This results in efficient energy transfer, and hence reduced noise, with acoustic characteristics that are faithful to that of an acoustic stethoscope diaphragm. The present invention discloses three basic embodiments: (a) A capacitive sensor, (b) a magnetic sensor, and (c) an optical sensor.
According to one aspect of the invention, there is provided a acoustic-to-electrical transducer for detecting body sounds, the transducer comprising (a) a capacitive to electrical conversion means, or (b) a magnetic to electrical conversion means, or (c) an optical (light) to electrical conversion means.
The capacitive to electrical conversion means comprises: a diaphragm having an electrically conductive surface, the diaphragm being mounted in a housing such that the diaphragm can contact a body for body sound detection; a conductive plate substantially parallel to the diaphragm, mounted within the housing, the conductive plate being positioned behind and spaced from the diaphragm to allow diaphragm motion, the diaphragm and conductive plate being connected in the form of an electrical capacitance to electrical circuitry; and a capacitance-to-electrical signal conversion means to convert capacitance changes to electrical signals.
The magnetic to electrical conversion means comprises a diaphragm that is placed against the body, the diaphragm having magnetic elements such as a permanent magnetic surface or electrically-induced magnetic field due to a wire or printed-circuit coil, so that a magnetic field is set up that is subject to change by motion of the diaphragm. The conversion means additionally comprises a magnetic field sensing means to convert the magnetic field changes to an electrical signal. Thus diaphragm motion affects the magnetic field, the magnetic field changes an electrical signal, and acoustic to electrical conversion is achieved.
The optical to electrical conversion means comprises a diaphragm placed against the body, with a light path that can be modified by motion of the diaphragm. A light source transmits visible or infrared light to the diaphragm. The diaphragm reflects the light, which is then detected by an optical detector, and changes in the reflected light signal due to diaphragm motion are then converted to an electrical signal. Another embodiment of the optical method is transmissive, with the light beam passing through an optical element that moves with the diaphragm, the motion of the optical element causing changes in the light beam received by the optical detector.
The present invention provides an acoustic-to-electrical transducer means for the detection of body sounds, such as for use in a stethoscope. The term xe2x80x9cbodyxe2x80x9d in this specification may include living or inanimate bodies. Living bodies may include humans and animals, while inanimate bodies may include, by example only, buildings, machinery, containers, conduits and the like. The sensor detects stethoscope diaphragm movement directly, converting the diaphragm movement to an electrical signal which is a measure of the diaphragm motion. Further amplification or processing of the electrical signal facilitates the production of an amplified sound with characteristics closely resembling the acoustic stethoscope sound, but with increased amplification, while maintaining low distortion. This is a significant improvement over the more indirect diaphragm sound sensing produced by the existing microphonic or piezoelectric methods described above. Since the diaphragm motion is sensed directly, the sensor is less sensitive to outside noise than the other methods described, and the signal is a more accurate measure of the diaphragm movement. In the case of the acoustic stethoscope, diaphragm movement produces the acoustic pressure waves sensed by the listener""s ears, and in the case of the present invention, that same diaphragm movement produces the electrical signal in a direct manner, the signal eventually being used to drive an acoustic output transducer such as headphones, to set up the same acoustic pressure waves impinging on the listener""s ears.
A fundamental advantage of the present invention is that diaphragm movement is not impeded by the acoustic-to-electrical conversion means, since there is a spacing between the diaphragm and other transducer elements. Therefore, the acoustic characteristics of the diaphragm are maintained, and the sound more closely resembles an acoustic stethoscope sound, which is familiar to the current user base of doctors, nurses and others. This is a unique aspect of this invention, in that other acoustic sensors do not require the amount of diaphragm motion required for a contact-type sensing device such as a stethoscope. Thus while other applications require only tens of microns of spacing, and the diaphragms typically move only a few microns when in use, this invention allows for movement of the diaphragm of more than 0.1 mm. Depending on the stiffness of the diaphragm, pressure against the body can result in 0.1 mm, 0.2 mm, 0.5 mm or even 1 mm of diaphragm displacement due to pressure.
The present invention discloses three sensing methods.
The first embodiment utilizes a capacitive sensing method. Capacitive acoustic sensors have been disclosed and are in common use in high performance microphones and hydrophones. However, the present invention uses the stethoscope diaphragm itself as one plate of the capacitive sensor which touches the body surface directly. This method of direct contact capacitive sensing of body sounds as described, is unique.
The sensor comprises a movable diaphragm with a conductive plane or surface, and a co-planar conductive surface (electrode or plate) placed behind the diaphragm, with a space or electrolyte between the two elements. The diaphragm""s conductive surface, in conjunction with the second conductive plate, form a capacitor. Movement of the diaphragm due to motion or sound pressure modulates the distance between the diaphragm and plate, producing a change in capacitance. One unique aspect of the invention lies in the fact that the stethoscope diaphragm forms one plate of the capacitor.
A feature of the invention is that the diaphragm, being the same element that makes contact with the body, is primarily sensitive to sounds emanating from the body, rather than sound transmitted through the air from ambient noise. By making contact with the body, the acoustic impedance of the sensor becomes matched to that of the body, rather than the surrounding air. Therefore, the capacitance change due to diaphragm motion is primarily due to body sounds, rather than overall ambient noise.
While a number of means are available for converting the capacitance variation to an electrical signal, the preferred embodiment performs this conversion by charging the capacitance formed by the diaphragm-plate combination to a high DC voltage, via a high resistance. This produces a somewhat constant charge on the capacitor. Movement of the diaphragm then produces a variation in the capacitance. If the capacitor charge is fixed, and the capacitance varies with time, a small AC variation in capacitance voltage is produced. This is sensed by a high-impedance amplifier, which is designed to detect the AC changes in capacitance voltage while avoiding rapid discharge of the capacitor.
A second method for detecting capacitance change is to employ the same diaphragm-plate capacitance in a high-frequency resonant or oscillation circuit, and detect changes in oscillation frequency produced by changes in the time constant of the capacitive circuit.
A third method of constructing a capacitive sensor, and sensing capacitance variation is via the use of an electret technique. This method requires that one or both of the plates of the capacitor formed by the diaphragm-plate be coated with a permanently charged material, such as an electret material, to create a permanent electric field between the plates. Since the plate, or plates, have a permanent electric field between them, the production of a high DC charge voltage is obviated, and voltage changes can be produced due to movement without the need for a DC charge voltage produced via a circuit.
A fourth method of constructing a capacitive sensor is to build the capacitive elements on a semiconductor substrate. In this case, the diaphragm contacts the body, there is a spacing for diaphragm motion, and the rear capacitive plate comprises the aluminum, copper or polysilicon conductive material as one of the layers of a semiconductor process. The fundamental principle of the invention still applies in that a diaphragm in contact with a body forms a movable capacitive electrode.
Any method of detecting capacitance change and converting such change to an electrical signal is encompassed by this invention. This invention therefore covers all such methods for detecting capacitance changes due to diaphragm motion.
It should be noted that while the preferred embodiment comprises a fixed plate behind the diaphragm, the invention includes methods whereby both plates are flexible and form a capacitance. In such a case, the basic principle applies whereby the capacitance varies due to sound pressure from the body, but the second plate is not necessarily rigid.
In the preferred embodiment, the fixed plate is mounted behind the diaphragm. In order to ensure acoustic isolation from external sounds, the fixed plate should preferably be mounted through a means which acoustically isolates it from the housing, or uses a means intended to prevent the fixed plate from vibrating. This is an important improvement which enhances noise isolation.
A variation of the basic principle of operation is to create two capacitors, by having the conductive diaphragm as described, with a conductive plate behind the diaphragm forming one capacitor, and a third plate behind the second, forming a second capacitor. The diaphragm and second plates are charged, while the third, rear plate is connected to an amplifier circuit. This two-capacitor method operates on essentially the same principle, whereby voltage across a charged capacitor varies in response to distance between plates, one plate being formed by the diaphragm. A further feature of the invention, is the method for constructing and producing the diaphragm. The diaphragm material must be flexible, and conduct electricity, in order to perform as a variable capacitor plate sensitive to sound pressure. This electrically conductive surface is preferably, but not necessarily, electrically insulated from the surface of the diaphragm that touches the body, for both safety and interference-prevention purposes.
A further feature of the preferred embodiment is the capacitive sensing circuitry connected to the diaphragm-plate capacitor. In the preferred embodiment, the circuit comprises two critical elements: (1) a high voltage DC bias generator with very high impedance, and (2) an AC amplifier with very high impedance to sense AC voltage changes without discharging the capacitor.
The invention also includes methods for signal amplitude control, DC charge voltage control to preserve battery power, and construction and manufacture of the capacitive sensor.
The first magnetic sensor embodiment of the invention comprises a diaphragm with permanently magnetized material adhered to or integral to the diaphragm, such that diaphragm movement results in changes in the magnetic field in the space behind the diaphragm. A magnetic field sensor is than placed at a distance from the diaphragm, but sufficiently close to detect changes in magnetic field due to diaphragm motion. The field sensor then converts magnetic field changes to an electrical signal. The diaphragm is housed such that it can be placed in direct contact with the body for body sound detection.
In another magnetic sensor embodiment, the diaphragm can be placed against the body, and has an electrical conductor on the rear side of the diaphragm such as a wire coil or printed circuit attached to the diaphragm or printed onto the diaphragm. A current in the coil sets up a magnetic field, or senses changes in a magnetic field produced by another coil or permanent magnet that is fixed behind the moving diaphragm. The diaphragm coil, or another magnetic field sensing means, converts changes in the magnetic field due to diaphragm motion to an electrical signal. Thus the coil can either produced the magnetic field and another circuit perform field detection, or the field can be produced by a separate magnet or circuit, and the diaphragm coil can perform field detection.
An optical sensor embodiment of the invention comprises a diaphragm which has optical elements, such as a reflective or transmissive plane integral to the diaphragm structure. A light transmitter, such as a laser or visible or infrared emitter is placed behind the diaphragm. A light sensor such as a photodiode or phototransistor is also placed behind the diaphragm such that it can detect the reflected light signal being modified by diaphragm motion. The sensor then converts the changing light signal to an electrical signal.
In one embodiment of the optical diaphragm structure, light from the emitter strikes the rear diaphragm surface. The surface or an underlying layer has a reflective pattern that produces either a pulsating or variable analog reflection signal that is then sensed by the optical detector and converted to an electrical signal.
In a second embodiment of the optical transducer, an optical structure such as a film is placed normal to the diaphragm plane, on the rear side of the diaphragm. The emitter and detector are placed such that the optical structure is within the light path between emitter and detector. The light path might be transmissive or reflective. In either case, diaphragm motion produces motion in the optical structure attached to the diaphragm, and the light signal is modified by mechanical movement of the diaphragm. This light signal is then converted to an electrical signal.
In all of the above embodiments, and others suggested by the invention, the diaphragm is physically separated from the conversion mechanism so that diaphragm movement is unimpeded. At the same time, the sensing means directly detects diaphragm motion in the form of a changing electric field, magnetic field, or optical signal. Thus the advantages of direct diaphragm sensing are achieved without the mechanical resistance of a mechanical sensor compromising acoustic characteristics of the diaphragm.